Computer-based drawing tools allow users to draw lines, artifacts, and other things on a display screen. In certain applications, a user desires to draw a line on a display screen for measurement or learning purposes, such as to ascertain the height or elevation of a previously drawn triangle or rectangle. In this case, the user might use a mouse or other input device to hand draw a line parallel with the bottom or base of a previously drawn triangle. However, the user typically is unable to draw a straight line using a mouse or touchscreen, and the resulting hand drawn line is not parallel to the base of the previously drawn triangle. In another example, a user may wish to draw a line perpendicular to a previously drawn line on a display screen. The above-mentioned difficulties with hand drawing straight lines will typically prevent the user from creating a hand drawn straight line that is truly perpendicular as desired. Accordingly, existing computer-based drawing tools are insufficient for many educational and other applications for drawing lines on a display screen.